Adjustable-gamma television signaltranslating stage



wanna epouv w la uam p s 4 u J. C. WILSON Filed March 2, 1940 auamng apouv -INVENTOR J HN G. WlLSON ADJUSTABLE-GAMMA TELEVISION SIGNAL-TRANSLATING STAGE Jan. 13, 1942.

ATTORN EY 26 mm 0 0 O 0 a o 5:?524 mymjmsfi 56:35 EEzSE 546M555 99E 0 ll 0 O my i N1 0.?

Patented Jan; 13, 1942 ADJ USTABLE- GAMMA TELEVISION SIGNAL- TRAN SLATING STAGE John C. Wilson, Bayside, N. Y., assignor to Hazeltine Corporation, a corporation of Delaware Application March 2, 1940, Serial No. 321,811

8 Claims.

It is frequently desirable in television systems to provide means for distorting video-frequency signals in a predetermined manner over predetermined portions of their amplitude range in order to procure apparent optimum fidelity of reproduction of scenes being transmitted and received. This distortion is desirable due to certain inherent characteristics of television signaltranslating apparatus as well as to certain physiological phenomena. More particularly, in conventional television signal-translating apparatus some of the component elements, such as amplifiers, cathode-ray tubes, and the like, have input-output r stimulus-response characteristics which deviate from linearity to such an extent as to result in appreciable distortion in the reproduced signal unless compensated for in some other portion of the system. Further, if the brightness of each element of a reproduced television image is proportional to the brightness of the corresponding point of the original image, the reproduced image may appear fiat and uninteresting to an observer. This may be due, in part, to the fact that the reproduced image ordinarily appears in black and white only and increased detail effects, such as are given to lighter portions of the picture by its natural color, are lost. This effect may also be due, in part, to the fact that both the contrast ratio and the average brightness of the reproduced image are generally considerably lower than the corresponding characteristics of the scene being transmitted.

Various devices have heretofore been proposed for adjustably distorting video-frequency signals so to reproduce television images that the contrast of either their lighter or darker portions is increased relative to that of the other portions. In general, such devices are subject to the limitation that adjustments thereof directly affect the over-all gain or contrast ratio of the system or the average brightness of the reproduced scene, or both. Also, it"is particularly desirable to provide an arrangement for adjusting the distortion to a desired value by means of a single simple circuit element, such as an adjustable resistor.

In photography, a film is said to have a "gamma which deviates from unity in accordance with differences in the relative detail of the film for the brighter and darker portions of the scene compared to other portions thereof. The same concept is applicable to television. The gamma of any signal-translating system may be defined as the slope of the input-output or stimulus-response curve of the system plotted on a logarithmic scale. Obviously, only where ganm'la is unity is there a linear relationship between the stimulus and response over the entire amplitude range' of the system.

It is an object of the present invention, therefore, to provide an improved method of, and apparatus for, eifecting a predetermined adjustable distortion of a video-frequency signal in a television system, whereby improved detail effects over a selected portion of the signal-amplitude range may be procured.

It is a further object of the present invention curing an over-all gamma in a television system which may be adjustable to an optimum value in accordance with the character of the signal being translated.

It is a still further object of the invention to provide an arrangement for procuring an overall gamma in a television system which is adjustable by adjustment of simple circuit elements and for compensating the usual tendency of the gain of the system to vary with such adjustment of the circuit elements.

In accordance with the invention, an adjustable gamma signal-translating stage in a television signal-translating system includes a vacuum tube having input electrodes and an anode circuit. Means are provided for applying a stabilized video-frequency signal to the input electrodes of the tube and means are provided, coupled to the tube, for deriving an output signal from the stage. An adjustable means is also provided for deriving, from the above-mentioned anode circuit of the tube, a signal voltage and applying the derived voltage to an electrode of the tube instantaneously to modulate the signals translated by the tube over the entire amplitude range thereof thereby to adjust the gamma of the stage, the applied voltage tending instantaneously to vary the gain of the stage over its amplitude range. The stage also comprises means, including the above-mentioned adjustable means, for compensating the tendency of the gain of the stage to vary with adjustments thereof.

For a better understanding of the present invention, together with other and further objects comprises a graph utilized to explain the operation or a portion of the circuit of Fig. 1; Fig. 3 is a circuit diagram 01' a modification of the portion of the system of Fig. 1 embodying the present invention; while Figs. 4 and 5 are graphs utilized to explain the operation of the circuit of Fig. 3.

Referring now more particularly to Fig. 1, the receiver there illustrated comprises a television receiver of the superheterodyne type including an antenna system Ill, Ii connected to a radiofrequency amplifier l2, to which are connected in cascade, in the order named, an oscillatormodulator I 3, an intermediate-frequency amplifier H, a detector and A. V. C. supply l5, a videofrequency amplifier indicated generally at l6 embodying the present invention, and a signalreproducing device H, such as a cathode-ray tube. A line-scanning generator I 8 and a fieldscanning generator I! are also coupled to the output of detector i5 and to the scanning elements of the signal-reproducing device. The stages i2-l9, inclusive, excepting the videofrequency amplifier it, may all be of conventional well-known construction so that detailed illustrations and descriptions thereof are unnecessary herein.

Referring briefly to the operation of the receiving system just described, television signals intercepted by the antenna circuit III, II are selected and amplified in the radio-frequency amplifier i2 and supplied to the oscillator-modulator i3 wherein they are converted to intermediatefrequency signals which, in turn, are selectively amplified in th intermediate-frequency amplifier' I4 and delivered to the detector I5. The modulation components of the signal are derived by the detector l5 and ar supplied to the videofrequency amplifier l6, wherein they are amplified and translated by the apparatus 01' the present invention, as will b presently further described, and from which they are applied to a brightness-control element 01' the signal-reproducing device H. The intensity of the scanning beam of device I! is thus modulated or controlled in accordance with the video-frequency voltages impressed upon the control element in the usual manner. A control-bias voltage developed by the A. V. C. supply l5 and proportional to the average carrier amplitude, independent of its light-modulation components, is supplied in the usual manner to control grids of tubes in one or more of the stages l2-i4, inclusive, for maintaining the signal-output amplitude of amplifier it within a relatively narrow range for a wide range of received signal intensities. The modulation signal is also applied to generators l8 and I9 and the synchronizing components of the signal are utilized therein to synchronize the operations of thes generators with corresponding apparatus at the transmitter. Saw-tooth current or voltage scanning waves are generated by the generators l8 and i9 and these waves are applied to the scanning elements of the device I I to produce scanning fields, thereby to deflect the scanning beams in two directions normal to each other so as to trace successive series of parallel lines or fields on the target of the signal-reproducing device to reconstruct the image.

Referring now more particularly to the portion of the system of Fig. 1 embodying the present invention, for the purpose or increasing the contrast of the fine detailed structure 01' certain portions of the reproduced image relative to that oi other portions thereof, the video-frequency amplifier l8 constitutes an adjustable gamma signal-translating stage designed in accordance with the present invention. Stage II comprises a vacuum tube 25 including, in the order named, a cathode. a first or control'electrode, a second or screen electrode, a third or suppressor electrode, and a fourth or anode electrode. A gridleak resistor 26 is provided for the input electrodes or vacuum tube 25 and the input electrodes are coupled directly to the output circuit of detector I5 so that a stabilized video-frequency signal is applied thereto. It will be understood that an unstabilized video-frequency signal can be derived from the unit preceding stage l6 oi the invention, providing additional backgroundreinsertion means are provided for stabilizing the applied signal at a given shade value of the translated signal at the input circuit of stage it. An adjustable degenerative resistor 21 is included in the cathode circuit of tube 25 and is common to the input and output circuits or the tube. while the suppressor electrode is connected directly to the cathode of the tube. Unidirectional operating potentials are applied directly to the screen electrode of the tub 25 and through a resistor 28 to its anode from the source +8. The load circuit of tube 25 comprises a resistor 29 and a coupling condenser I0.

Reference is made to 18. 2 for an explanation 01' certain of the operating characteristics of the adjustable gamma signal-translating stage ll of Fig. 1. For the purpose or explanation, it will be assumed that an input signal or the wave form of curve A of Fig. 2 is applied to stage it and that the stage has the grid voltage-anode current characteristics indicated by curve B, 11 the value of resistor 21 is reduced to zero. It now it is assumed that resistor 21 is caused to have some finite value, it is seen that the difference in potential between the third or suppressor electrode and the second or screen electrode of tube 25, which is at the fixed unidirectional operating potential of +3, is caused to vary in accordance with the signals translated by the stage, due to the voltage drop across cathode resistor 21. Thus, the suppressor electrode acts as a control grid upon the virtual cathode which is formed between the screen electrode and the suppressor electrode and serves to augment the amplification or the instantaneous signal input to signaltranslating stage it directly in accordance with the amplitude thereof. The operating grid voltage-anode current characteristic of the stage thus differs from the static characteristic of curve B and may have a form such as is illustrated by curve C of Fig. 2 for a particular setting of adjustable resistor 21. It is apparent that the signal is translated with a diflerent gamma if the tube has the operating characteristic of curve C than if it has that represented by curve B; that is, the incremental differences between shade values in the low amplitude portion of the signal are relatively increased so that. with the present conventional television signal, the fine detail or the darker portions of the reproduced image is emphasized.

Also, it is seen that the gain of the stage I tends to increase with the adjustment of the gamma-controlling action described which, in general, is undesirable inasmuch as it is usually desired to eifect a gamma adjustment without an appreciable variation of the over-all gain of the system which affects the over-all contrast of the reproduced picture. However, resistor 21 is coupled in the circuit of tube as a degenerative resistor; that is, it is a cathode resistor common to the input and output circuits of tube 25 and thus is effective to provide degeneration in a manner well understood in. the art which, by a proportioning of circuit elements, may be made to compensate the tendency of the gain of the stage to increase when its gamma is adjusted in the manner described by increasing the value of resistor 21. In summary, therefore, the resistor 21 is included in an adjustable means for deriving from the anode circuit of tube 25 a signal voltage and applying the derived voltage to an electrode of the tube, specifically, to the suppressor electrode of the tube, instantaneously to modulate the signals translated by the tube over the entire amplitude range thereof, thereby to adjust the gamma of the stage. The voltage applied to the suppressor electrode of tube 25 tends instantaneously to vary the gain of the stage including tube 25 over its signal amplitude range and the connection of resistor 21 as a degenerative cathode resistor for tube 25 provides means, including the adjustable resistor 21, for compensating the tendency of the gain of the stage to vary with adjustments thereof.

The modification of the invention illustrated in Fig. 3 is generally similar to that of Fig. 1 and it will be understood that the adjustable gamma signal-translating stage 36 of Fig. 3 can be connected into the receiver circuit of Fig. 1 by connecting correspondingly marked terminals of Fig. 3 to the receiver of Fig. 1. Similar circuit elements in the two figures have identical reference numerals. The signal-translating stage 36 comprises a vacuum tube including, in the order named, a cathode, a first or control electrode, a second or screen electrode, an anode, and a secondary electron-emitting electrode in the circuit of which is included an adjustable resistor 38. The tube 35 may be of the British type TSE4.

Reference is made to Figs. 4 and 5 for an explanation of the operating characteristics of the adjustable gamma signal-translating stage 36 of Fig. 3. The characteristic curve D of Fig. 4 illustrates the secondary electron-emission ratio voltage characteristic of the secondary electronemitting electrode of tube 35. If it is assumed that the value of resistor 38 is reduced to zero, the operation of circuit 36 is conventional and the primary electrons from the cathode of the tube 35 incident upon the secondary electronemitting electrode of the tube cause th emission of secondary electrons which are collected by the anode of th tube to produce an output signal across load resistor 29. It will be assumed that the characteristic curve E of Fig. 5 represents the grid voltage-anode current characteristic of tube 35 under the conditions assumed.

If the adjustable resistor 38 is now given a finite value, it is seen that the potential of the anode of tube 35 is increased with reference to the potential of the secondary electron-emitting electrode, thus tending to increase the secondary electron current collected by the tube anode, and thus the signal output of the tube, so that the grid voltage-anode current characteristic of the tube tends to assume the form of curve F of Fig. 5. However, simultaneously with the reduction of the potential of the secondary electron-emitting electrode of tube 35, if it is assumed that the tube is operating upon the portion V1, V: of the characteristic curve D of Fig. 4, the ratio of primary electrons to secondary electrons is decreased, thus tending to decrease the output of the tube effectively to compensate for the tend ency of the gain of the stage to vary with increase in value of resistor 38, Adjustment of the gamma-controlling resistor 36 thus tends to vary the gain of stage 36 and the arrangement comprises means including the resistor 38 for compensating the tendency of the gain of the stage so to vary.

While there have been described what are at present considered to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modi fications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. In a television signal-translating system, an adjustable gamma signal-translating stage including a vacuum tube having input electrodes and an anode circuit, means for applying a stabilized video-frequency signal to said input electrodes, means coupled tosaid tube for deriving an output signal from said stage, an adjustable means for deriving from said anode circuit a signal voltage and applying said voltage to an elee trode of said tube instantaneously to modulate the signals translated by said tube over the entire amplitude range thereof, thereby to adjust the gamma of said stage, said voltage tending instantaneously to vary the gain of said stage over its amplitude range, and means including said adjustable means for compensating said tendency of the gain of said stage to vary with adjustments thereof.

2. In a television signal-translating system, an adjustable gamma signal-translating stage including a vacuum tube having input electrodes and an anode circuit, means for applying a stabilized video-frequency signal to said input electrodes, means coupled to said tube for deriving an output signal from said stage, means including an adjustable resistor in said anode circuit for deriving a voltage and applying said voltage to an input electrode of said tube instantaneously to modulate the signals translated by said tube over the entire amplitude range thereof, thereby to adjust the gamma of said stage, said voltage instantaneously tending to vary the gain of said stage over its amplitude range, and said means including said adjustable resistor including means for compensating said tendency of the gain of said stage to vary with adjustments thereof.

3. In a television signal-translating system, an adjustable gamma signal-translating stage including a vacuum tube having input electrodes and an anode, means for applying a stabilized video-frequency signal to said input electrodes, means coupled to said tube for deriving an output signal from said stage, means including an adjustable degenerative resistor coupled in circuit with the said anode for instantaneously modulating the signals translated by said tube over the entire amplitude range thereof, thereby to adjust the gamma of said stage, said last-named means tending instantaneously upon adjustment there- 4 steam of to vary the gain of said stage over its amplitude range, and means including said degenerative' resistor for compensating said tendency of the gain of said stage to vary withadjustments thereof.

- '4.'In a television signal-translatingsystem, an adjustable gamma signal-translating stage including a vacuum tubehaving :input electrodes riving rrsmtneeircuit of immune new.

emitting electrodes. voltage and applying said voltage to 'saidsecondary electron-emitting elec-' trode instantaneously-to modulate the signals and an anode circuit, means for applying a sta=-- bilized video-frequency signal'to said input electrodes; means coupled to said tube for deriving an output signal from said stage, an adjustable resistor in said anode circuit for deriving from said anode circuit a voltage and applying said voltage to an electrode of said tube instantaneously to modulate the signals translated by said tube over the entire amplitude range thereof, thereby to adjust the gamma of said stage, said voltage tending instantaneously to vary the gain of said stage over its amplitude range, and means including said resistor coupled in circuit with said input electrodes for compensating said tendency of the gain of said stage to vary with adjustments thereof.

5. In a television signal-translating system, an

adjustable gamma signal-translating stage including a vacuum tube having input electrodes, a screen electrode, an anode electrode, and a suppressor electrode between said screen electrode and said anode electrode, mean for applying a stabilized video-frequency signal to said input electrodes, means including a circuit coupled to said anode electrode for derivingan output signal from said stage, an adjustable means for deriving from said anode circuit a voltage and applying said voltage to said suppressor electrode of said tube instantaneously to modulate the signals translated by said tube over the entire amplitude range thereof, thereby to adjust the gamma of said stage, said voltage tending instantaneously to vary the gain of said stage over its amplitude range, and said adjustable means v including means for compensating said tendency of the gain of said stage to vary with adjustments thereof.

6. In a television signal-translating system, an adjustable gamma signal-translating stage including a vacuum tube having input electrodes, a secondary electron-emitting electrode, and an anode, means for applying a stabilized videofrequency signal to said input electrodes, means coupled to said anode for deriving an output signal from said stage, an adjustable means for detranslated by said tube over the entire amplitude range-thereof, thereby to adjust the gamma of said "stage, said voltage tending instantaneously to vary the gain of said stage over its amplitude range. and means for utilizing said derived voltage for varying the relative potential of said anode and said secondary electron-emitting electrode for compensating said tendency of the gain of said stage to vary with adjustments thereof.

7. In a television signal-translating system, an adjustable gamma signal-translating stage including a vacuum tubehaving input electrodes and a secondary electron-emitting electrode, means for applying a stabilized video-frequency signal to said input electrodes, means coupled to said tube for deriving an' output signal from said stage, means including an adjustable resistor connected in circuit with said secondary electron-emitting electrode for instantaneously modulating the signals translated by said tube over the entire amplituderange thereof, thereby to adjust the gamma of said stage, said gammaadiusting means tending instantaneously to vary the gain of said stage over its amplitude range, and means including said adjustable resistor for compensating. said tendency of the gain of said sistor in circuit with said secondary electronemitting electrode for instantaneously modulating the signals translated by said tube over the entire amplitude range thereof, thereby to adjust the gamma of said stage, said gammaradjusting means tending to vary instantaneously the gain,

of said stage over its amplitude range, and means including said adjustable resistor for compensating said tendency of the gain of said stage to vary with adjustments thereof.

JOHN cI WILSON. 

